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Got a call from the in laws. Their furnace has been acting weird so they called someone in. The guy checked things, tightened a loose wire (apparently the continous fan had quit running a couple of weeks ago) and came to the conclusion that the thermostat is on the fritz and they need a new one. They have an 80% two stage Carrier and the thermostat (Carrier) is a simple heat/cool mercury stat. The set up is all of three years old (heat & cool). Apparently, the temp in the house gets uncomfortably cool before the heat kicks in and then it heats past the set point before shutting down. I did notice today that the anticipator was set at ".3" which I think is pretty standard.

When they told me what the tech said about needing a thermostat, I kinda went "huh?". The thing is so simple I'm thinking "what can go wrong with it?". I took the cover off of the thermostat to have a look. The bimetallic twists back and forth, the mercury goes back and forth...what could be the matter?

Just wondering, what made him determine that the Stat was on the fritz? If he found a loose wire for constant fan and that fixed the problem they were having......... why replace a Stat that has nothing wrong with it? Mercury Stats aren't like digitals in the manner that if your set to 70 and you hit 70 click it stops calling. Mercury has to roll, that tech should know that. Or did I miss something in your post?

Ahhhh! See, I did miss that one beenthere! Yes your stat should be for 2 stage. You should just be able to go and purchase a sub base for 2 stage though to modify the stat. I'm assuming the stat is also 3 years old of course... Otherwise just purchase a stat that is 2 stage.

Well, it was the stat. My bro in law sent one of his techs over and he determined it was the stat as I mentioned in the first message. So, they bought a VisionPro 8000 on advice of the tech and lucky me...I got to install it. Things are running like a top, now. But I'm a bit disillusioned that the tech didn't agree that a two stage stat was the optimum way to go. Oh well, the tech has a two stage furnace and uses the VP8000 on his system and says it's a good combination. But hey...who am I to argue with a trained HVAC technician about what the right equipment for the job might be?

Hmmm...so much for things being ducky. The folks just called. The furnace is up to its old tricks. Apparently, the stat is set for 22 and the room temp is now 18. I guess the tech will have to take a harder look at what the problem is.

Not all 2 stage furnaces need 2 stage stats.The carrier unit is one that doesn't require the 2 stage stat.There is a dip switch option on the control board.If it's set for 1 stage only,it won't hit 2nd stage,if its set for 2 stage,the board will call for the 2nd stage heat if needed.

I'm not familiar with the vision pro stat at all.Is it also a mercury stat?I know in connecticut,you cant install mercury stats anymore.
Has the t-stat always overshot by 3 degrees?

The furnace will run longer if left on 1st stage heat.2nd stage comes on after a certain amount of time if the t-stat isn't satisfied or if you set the t-stat for more than 2 degrees above room temp.So if you want your humidifier to run longer,leave it on 1st stage.

Well...looks like they bought a new thermostat for nothing. The tech came back and after some more serious probing, discovered that the pvc drain for the humidifier was plugged. This caused all of the humidifier water to back up into the pan for the evaporator and consequently, flood over right onto the burners. Tech replaced the pvc drain with clear flex tube and pulled and dried all of the burners. Apparently now the furnace is running well.

Now, my technician friend is telling me to tell my in laws to make sure the furnace was inspected very well as water damaged units are "suspect to improper operation and replacement is recommended".